ETFE – material with perspective

ETFE - ethylene-tetrafluoroethylene - a material that has found a new place in contemporary architecture in the form of films. Simply put, we can imagine it as a thicker, rigid plastic wrap, but in reality, it is a material with high utility properties, primarily strength and durability.

Architects searching for a lightweight and transparent type of roofing for spaces with large spans at some point exceeded the technical possibilities of using glass. At that moment, when transparent roofing using glass crossed the technical or financial boundary for a given project, it was necessary to replace heavy laminated glass and the resulting massive supporting structure with another material.
An excellent option was the use of transparent films, which due to their lightness only required a much more delicate supporting structure.
The films appeared primarily in the roofing of atriums and other public spaces, in sports buildings, or transportation buildings such as airports.

Their greatest advantage is high transparency (95-100%) and simultaneously, the permeability for UV radiation, which glass does not have. UV radiation components beneficial for health pass through (component A 100% and B 50%), while the harmful UV C component is fully filtered out. This interesting property is utilized especially where UV radiation is desirable - botanical and zoological gardens, recreational areas such as swimming pools.
ETFE films are used in almost all cases in the form of inflatable cushions, whose stability is ensured by pre-stressing with compressed air. The overpressure is relatively small (0.3-0.8 kPa), but unlike pneumatic structures, they do not lose stability when the pressure drops, only the shape (the roofing function is preserved due to attachment to the edge profile).

The overpressure also ensures a spatially curved shape, advantageous for wind and snow loads, and creates a highly smooth surface that does not allow dust to adhere to the roofing; cleaning debris simply requires rain and snow. This maintenance-free technology is another significant advantage of ETFE films.

Compared to double-glazing, they have somewhat lower thermal insulation properties - the U-value ranges between 1.18 - 2.95 W/m²K, which is influenced by the number of layers or air chambers in the cushion.

U-value:	2-layer film (1 chamber)	2.95 W/m²K
U-value:	3-layer film (2 chambers)	1.96 W/m²K
U-value:	4-layer film (3 chambers)	1.47 W/m²K
U-value:	5-layer film (4 chambers)	1.18 W/m²K

From a mechanical perspective, the films have thicknesses ranging from 0.05 to 0.2 mm and have approximately 1/5 the strength compared to woven coated fabrics, but their advantage lies in very low surface weight, which ranges from 175-400 g/m².
Tensile strength of min. 40N/mm allows for spans of individual cushions between 3.5 and 5 m, while the length is practically unlimited in terms of strength. These spans can be further increased by supporting the lower layer with a cable net.
For comparison with glass roofing, tests on hail resistance were conducted in Switzerland, where ETFE films received certification as "hail-resistant material." However, there have been past instances of extreme hail, where surrounding glass roofs were totally destroyed and falling debris could threaten the spaces below; the film roofing, although damaged, did not lose its roofing function, and the damaged parts could be repaired when weather conditions improved.

ETFE films are manufactured in clear or opaque forms, but there are additional options for restricting sunlight passage or shading. The first of these is printing on one or more layers. The color of the print currently exists only in gray (silver), but the shape of the print is practically unlimited. An interesting variant is the inverse printing of 2 layers. A technically quite simple system regulating pressure in the upper and lower chambers brings the central membrane closer or further from the upper printed layer, thereby thickening or thinning the printed surface. Thus, the degree of shading can be regulated relatively smoothly.

Another shading option is the insertion of a shading textile between the ETFE layers. A retractable "roller" was even used at a gas station in Munich, Germany, integrated between the layers of the cushion.

In terms of fire safety, ETFE films are classified according to DIN 4102 as class B1. An important property, however, is that they do not drip burning particles, and at temperatures above 270°C, the material practically sublimates and at that moment can replace the equipment for heat and smoke removal.

The lifespan of the material is also a common question for investors. ETFE in the form of film cushions has been in use since the 1980s. Laboratory tests indicate a verified lifespan of more than 30 years, but I have personally verified that the material shows no optical deviations from a new sample after approximately 30 years of use.

Used materials: Construction magazine, Stadiums section; Detail; materials by Ceno Tec GmbH; materials by Foiltec Ltd; Book Roofs (Detail); own archive

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ETFE - examples of implementations

One of the implementations that gained popularity due to the recent World Cup is the Munich Allianz Arena. The stadium with a capacity of 66,000 spectators is clad with 2,760 inflatable cushions made of ETFE film. The diamond-shaped cushions ranging in size from 7.6 to 40.7 m² create an incredible area of 66,500 m²! The main feature of the stadium is the color variability of the façade - when Bayern plays, the stadium shines red, and when TSV 1860 is the home team, the stadium lights up blue. The construction cost of 340 million EUR is a price that is not within the budget of every club...

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Beijing, stadium for the 2008 Olympic Games

The oval structure, reminiscent of a bird's nest, consumed over 40,000 tons of steel to create a structure for 40,000 m² of ETFE film, protecting visitors from rain, snow, and sandstorms.
The building is topped with a roof made of PTFE fabric covering nearly 50,000 m².

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One of the pioneering buildings made of film cushions is the Eden Project, near Cornwall, England. The hexagonal cushions have a diameter of about 8 m, and their substructure is almost invisible. The original consideration for a glass shell failed precisely due to the bulkiness and the unreasonable cost of the steel supporting structure.

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Tropic Island near Berlin - the former factory hall for the production of airships from CargiLifter AG has recently gained popularity due to its conversion into an indoor swimming pool, or rather an entertainment center with a sandy beach and swimming with the illusion of a tropical sea. The covered space with a volume of an incredible 5.5 million m³ and a clear height of approximately 99 m could rather be called a covered and temperature-controlled outdoor space.
The artificial landscape is divided into several zones - tropical village, Bali lagoon, south sea, and tropical rainforest covering about 12 m high hill in the center of the hall.
In searching for a material for the new roofing of the southern central part of the hall, properties such as high transparency, the possibility of UV radiation passage, and primarily the use of the original structure of the hall, which was covered with opaque fabric, were required. When creating the concept, materials such as glass, polycarbonate, fabric membranes, and various films were considered. After weighing all the pros and cons of individual solutions, a tri-layer ETFE film (composed of 0.2 - 0.1 - 0.2 mm) was chosen.
In the competition for the final roofing solution, the solution provided by the German company Ceno Tec GmbH was selected, consisting of large space, double-chamber cushions, which are supported on the underside by a cable net made of parallel 16 mm diameter cables, and on the upper layer, a net of 14 mm diameter cables, tensioned between frames and the gutter cable with a diameter of 50 mm (this was used from the original roofing). Statistically, this structure acts as one large space cushion, whose upper and lower layers are just the cable nets, and the ETFE cushions serve as their substructure - the internal structure ensuring the shape and stability of the whole. Each field consists of a total of 14 cushions with an area of approximately 350 m² (2 columns and 7 rows) measuring about 17x20 m.
The permanent overpressure in the cushions of 0.3 kPa can increase to 0.8 kPa under snow load.

Another major issue to be resolved was building physics. To maintain transparency, it was necessary to eliminate the formation of condensation inside the cushions while simultaneously
Thanks to interactive computer simulation and subsequent adjustments of the shapes of the cushions, it was possible to maintain the U-value below 1.4 W/m²K under extreme conditions (December, January).

Thanks to the roofing made of ETFE films with a total area of 20,000 m², visitors at the beach can indeed sunbathe, and live plants enhance the atmosphere of a Thai or Indonesian vacation.